In several parts of the world the inch system has retained its standardized state, where the so-called "technical inch" is used as measuring unit for specifying data of technical dimensions, lengths. The exact value of this "technical inch" expressed in millimeters is 25.4 mm. Component parts with dimensions given in inches, e.g., ball bearings being used, stored and even manufactured--at least for replacement purposes--in countries where only the metric system is standard.
Dimensions smaller than one inch are expressed as fractions of the inch: x/2, x/4, x/8, x/16, x/32, x/64, x/128, x/256, x/512, etc. This system of expressing dimensions does not allow the use of arbitrary intermediate sizes, especially in the range of zero to 1/128". A further deficiency of this unit system results form the incapability of computers (functioning in decimal system) of performing arithmetic operations--e.g., finding the sum of a series of dimensions--without converting the dimensions into the decimal (metric) system, and after this conversion, the resultant metric value has to be re-converted into the inch system. This conversion to the decimal system has to be performed with an accuracy of at least six decimals in order to prevent--in the course of rounding the obtained data--occurrence of errors of magnitude exceeding the order of 1/100 millimeters (see the table of FIG. 1). Computerized registering and arranging of component parts in the series of rounded sizes are also more problematic than that of sizes expressed in millimeters, and can only be done in a system specifically developed for this purpose. Trouble is caused also when component parts suitable for mutual replacement but having dimensions given in different unit systems are to be compared and matched. No satisfactory solution can be found when this comparison is to be made by converting dimensions given in inches to millimeter dimensions, because the results appear as eight-digit decimal fractions, which are difficult to keep in mind, difficult to handle and are not round whole numbers.
It is an everyday task to compare inch dimensions with those given in millimeters. For this comparison almost invariably a calculator is used, multiplying the dimension concerned with the respective conversion factor, just as the gauge used for this purpose is almost exclusively a slide caliper provided with a scale graduated both in inches and millimeters.